In these years, there have been used an extremely large amount of plastics the waste of which has induced global environmental problems, for example, short of lands for filling-up, worsened views, threat to marine organisms and environmental pollution. The so-called "commonly employed resins" include polyethylene, plypropylene, polystyrene, polyvinyl chloride, polyethylene terephthalate, etc. These resins have been disposed by incineration or filling-up.
However, these disposal treatments suffer from some problems. When resins such as polyethylene, polypropylene and polystyrene are incinerated, incinerators are frequently damaged due to high incineration calory thereof and thus become useless within a short period of time. Although polyvinyl chloride has a low incineration calory, it is known to liberate a harmful gas when burnt. When filled-up, these commonly employed resins are not degraded but remain as such semipermanently owing to the high chemical stability, which results in the serious shortage of lands for filling-up.
When these resins are carelessly thrown away in nature, they damage the beauties of nature because of the high chemical stability. Moreover, they bring about the death of marine organism, birds, etc. taking them by mistake, thus reducing the valuable biological resource. That is to say, the disposal of these plastics is one of the causes of the environmental pollution. To solve these problems, studies have been vigorously made on biodegradable polymers. These biodegradable polymers having been attracted public attention involve so-called "lactic acid-based polymers", i.e., polylactic acid and copolymers thereof. Different from common plastics, these lactic acid-based polymers are completely degraded finally into water and carbon dioxide.
Further, these lactic acid-based polymers have low incineration calory and, therefore, never damage incinerators. Furthermore, they liberate no harmful gas when burnt. Moreover, they can be obtained from vegetable materials which can be easily reused. Thus, there arises no problem of drying up the petroleum resources. Accordingly, lactic acid-based polymers having these advantages are expected as useful substitutes for the commonly employed resins.
There have been employed films and sheets produced by processing paper and synthetic resins and aluminum foils in packaging or packing various foods, drinks, drugs, miscellaneous goods, etc. in liquid, powdery or solid forms. In particular, films and sheets are employed packaging or packing bags, cases or thermoformed lightweight containers for various purposes, since they have a number of favorable characteristics such as high waterproofness, transparency, strength and thermoforming properties and low cost. It is required that these packaging or packing materials have, in particular, good heat-sealing properties and high heat resistance.
Films and sheets made of synthetic resins are processed into various bags and cases by folding and adhering at least one side thereof by taking advantage of the heat-sealing properties of the resins. Also, films and sheets are formed into lightweight containers, in which foods, drinks, drugs, miscellaneous goods, etc. are packed with the use of the rigidity of the contents, by various thermoforming methods such as vacuum forming, vacuum pressure forming, hot plate pressure forming and deep drawing vacuum forming.
After packing with the contents, these containers are often sealed at the opening by adhering films, sheets or lids formed by thermoforming these films, sheets, etc. That is to say, synthetic resin films and sheets are processed by various methods by taking advantage of the heat-sealing properties thereof and put into practical uses. In these cases, the adhesion power and appearance of the heat-sealed parts are important factors.
When the heat resistance during storage and transportation is taken into consideration, it is necessary that these containers, films and sheets are durable to high temperatures of at least 60.degree. C. in usual. On the other hand, lightweight containers in which heated contents such as hot foods are to be packed (for example, food packs for rice, fried foods, prepared foods, etc., hot-fill containers for jam, pudding, jelly, etc.) should be resistant to high temperatures of 80.degree. C. or above. In addition, these containers should be heat-sealable, since they are sealed after putting the contents thereinto.
To solve the problem of heat resistance, U.S. Pat. No. 5,076,983 discloses a method for elevating the heat resistance which comprises thermosetting a polylactic acid stretch film at 130.degree. C. for 1 minute to thereby reduce its degree of shrinkage in boiling water from 66% to 4%. However, this method suffers from a problem that this film cannot be heat-sealed since it has been already crystallized.